An optical fiber cable includes a sheath system which protects an optical fiber which extends along the longitudinal axis of the cable and which serves as an optical communications path. Not only does the sheath system protect the glass fiber, but also it provides the cable with flexibility and with tensile, flexural and impact strength. For multi-fiber cables, the sheath system may include several extruded layers of plastic as well as one or more metallic shields disposed between elements of the sheath system.
Single fiber cables are well known in the art. They may be terminated with biconic connector plugs such as those shown in U.S. Pat. No. 4,512,630 which issued on Apr. 23, 1985 in the name of P. Runge. Such cables are used, for example, in central offices to connect cables to optical transmission apparatus.
Generally, a single fiber cable includes a buffered optical fiber. A buffered optical fiber includes a coated optical fiber which is enclosed in a buffer layer in which the buffer layer engages the coated optical fiber. The buffer layer typically is made of an extruded plastic material such as polyvinyl chloride. Over the buffered optical fiber is a yarn which provides strength for the cable and which is enclosed by a plastic jacket. The yarn may be an aramid fibrous yarn and is usually served in a helical fashion about an advancing buffered optical fiber.
One problem with single fiber cables is the maintenance of concentricity between the fiber and the served yarn, particularly during the manufacturing operation. When it is off-center, the buffered fiber has a tendency to conform to the helical path of the served material which may result in microbending of the optical fiber. Microbending may result in a significant loss in transmission through increased signal attenuation. Also, when concentricity is not maintained, it becomes more difficult to connect the single fiber cable so that the optical fibers are aligned precisely. Concentricity of an optical fiber and its coverings makes handling during manufacturing easier, increases the durability of the product and facilitates more precisely aligned connections.
One solution to this problem is to replace the served yarn with an extruded tube, but this solution is a costly one. Instead, the strength members can be served directly over a heavily buffered fiber. The heavy buffering reduces the tendency for the helical serving to induce microbending in the optical fiber. However, the heavy buffering increases production costs without improving the concentricity of the fiber within the serving.
Another solution to the loss of concentricity during manufacture is to keep the fiber taut during the serving operations. If sufficient tension is placed on the buffered fiber as the serve is being applied, and if this tension is maintained during all subsequent operations, the fiber can be maintained in its center position. However, the tension required to do this would be likely to damage or break the buffered optical fibers. Also, residual tension in the optical fiber after cable manufacture can itself result in higher attenuation and poorer mechanical performance. In U.S. Pat. No. 4,441,787, an optical fiber is maintained concentrically within a textile serve by the use of a highly viscous coating which is applied to the optical fiber. The viscous fluid such as, for example, a colloid, is applied over the coated fiber prior to the serving of the yarn.
Other problems exist in the presently used single fiber cables. One relates to size. Because of the buffering, the cross section of the fiber is relatively large. Increased size cables require more space in central office wiring and more time to remove the coverings to access the optical fiber far connectorization. Also, the stress-strain curve for presently used cables, wherein a plurality of strength member yarn are served or wrapped helically about the buffered optical fiber, includes a knee. This occurs because of the tendency of the cable to elongate somewhat prior to the yarn becoming straightened and loaded by tensile loads imparted to the cable. As a result, the optical fibers enclosed by such yarns are stressed which may result in damage to the optical fiber. Further, it has been found that the single fiber cables are somewhat difficult to strip for connectorization. This is particularly true in those instances where it is desired to expose a substantial length of optical fiber for particular connectorization arrangements.
Further, the buffered optical fiber may be used by itself without being incorporated into a cable structure. In order to provide suitable strength for such an optical fiber, it is not uncommon to enclose it in a layer of a plastic material such as, Hytrel.RTM. plastic, for example, which has greater strength than polyvinyl chloride. Although the use of some other plastic materials provide a solution for the strength problem, they may be somewhat difficult to remove from the optical fiber during connection procedures.
What is needed and what seemingly is not provided by the prior art is a buffered optical fiber which is small in size, and which does not elongate prior to loading of the strength members therein. Further, the sought-after buffered optical fiber should be one in which the engagement of the plastic buffer layer with underlying materials may be varied to meet particular customer requirements. For example, in one use, it is desired that the covering materials may be removed easily to expose the optical fiber for connectorization. Still further, the sought-after cable should be one which overcmes the prior art problem of microbending caused by helically applied yarn and non-concentric fiber covering.